1. Field of the Invention
This invention relates to ester based, in particular diester and polyol ester-based turbo oils which exhibit superior antioxidancy and reduced deposit forming tendencies. More particularly it related to turbo oils comprising esters of pentaerythritol with fatty acids as basestock, and containing a combination of additives which impart improved antioxidancy and reduced deposit formation.
2. Description of the Related Art
Organic compositions such as mineral oils and lubricating compositions are subject to deterioration by oxidation and in particular are subject to such deterioration at high temperatures in the presence of air. This deterioration often leads to buildup of insoluble deposits which can foul engine parts, deteriorate performance, and increase maintenance. This is particularly the case for lubricating oils used in jet aircraft where wide temperature ranges and extreme operating conditions are likely to be encountered. Proper lubrication of aircraft gas turbines, for example, requires the ability to function at bulk oil temperatures as low as -65.degree. F. to as high as 450.degree.-500.degree. F.
Most lubricants contain additives to inhibit their oxidation. For example, U.S. Pat. No. 3,773,665 discloses a lubricant composition containing an antioxidant additive mixture of dioctyl diphenylamine and a substituted naphthylamine. U.S. Pat. Nos. 3,759,996; 3,573,206; 3,492,233, and U.S. Pat. No. 3,509,214 disclose various methods of oxidatively coupling alkylated diphenylamines with substituted naphthylamines.
Patents disclosing the use of tri-substituted triazines in lubricants generally demonstrate the antioxidant function of these molecules when either used alone, or in combination with other antioxidants. They do not describe the use of these materials as anti-deposition additives. U.S. Pat. No. 3,250,708 describes the use of several triazine derivatives, and combinations with hydroxyl aromatic co-antioxidants. U.S. Pat. No. 3,278,436 and U.S. Pat. No. 3,322,763 describes tri-substituted triazines including piperidinyl bridged triazines in combination with hydroxyl aromatics.
European Patent application 002,269 discloses the use of tri-substituted triazines where at least one of the amino substituents contains at least one hydrogen as antioxidants, and in combination with arylamine antioxidants.
U.S. Pat. No. 3,642,630 discloses that the use of symmetrical and asymmetrical substituted triazines with N-substituted phenothiazine imparts good oxidation stability to synthetic ester based lubricants over a wide range of temperatures.
Other triazine derivatives disclosed in a number of patents to stabilize oils would not be suitable for use in aviation turbine oils as these derivatives contain halogens which are corrosive to metals. For example, U.S. Pat. No. 3, 198,797 utilizes 2,4-dichloro-6-dialkyl-dyhydroxy-anilino-1,3,5 triazines. Similarly, U.S. Pat. No. 3,202,681 utilizes monohalogen substituted triazines, especially monochloro substituted ones.
It has now been discovered that the deposit forming tendencies and antioxidant properties of these basic antioxidant systems, e.g., tri-substituted triazines alone or in combination with arylamines, can be greatly enhanced by the addition of a small amount of a non-antioxidant, sulfur containing additive, specifically trithiocyanuric acid.